Three
earthquakes, ranging in magnitude from 7 to 8, shook the ground near New
Madrid, Mo., in 1811 and 1812, causing the ground to roll in visible waves
and ringing church bells as far away as Boston. The temblors signaled a
previously unknown seismic zone and today the Reelfoot scarp still bears
its witness: a five-meter-high, 32-kilometer-long slope in northwest Tennessee.
Uplifted by the Reelfoot blind-thrust fault in the New Madrid seismic zone,
the scarp warns of earthquakes to come.

Karl Mueller of the University of Colorado at
Boulder and his colleagues published work in the Nov. 5 Science calculating
the slip rate of the fault by measuring the relief across the scarp, nine
meters vertical distance, the strike, and a dip of 55 degrees. Combining
the measurements with the age of the folded

Oblique shaded relief image created
from a digital elevation model of the Reelfoot scarp, looking west.
The landscape is illuminated from the north. Adam Bielecki,
Research Systems, Inc. in Boulder, Colo.

sediment, about 2,300 years, they determined the
fault’s rate of movement to be about 5 millimeters a year.Their calculations
indicate the Reelfoot fault has stored up enough energy to release a “Northridge-sized
earthquake,” Mueller says. The 1994 Northridge quake, also the result of
a blind-thrust fault that did not break the earth’s surface, created the
most destructive ground movement recorded in Southern California—killing
63 people and causing over $13 billion in damage.

Although no one can predict when or how the energy
in the New Madrid region will be released, Mueller’s study contrasts with
one that appeared in the April 23 Science and questioned New Madrid’s future
risk. “People in Missouri should be as prepared as they are in Southern
California,” Mueller says. The USGS estimates a quake of magnitude 6 to
7 as having over a 90 percent chance of happening in the area in the next
50 years. When that happens, residents throughout the central and eastern
United States will likely feel its effects.